The present invention relates in general to seatbelt pretensioners and, more particularly, to a pretensioner that incorporates a soft-start piston actuator.
Seat belt pretensioners remove slack from a seat belt in the event of a collision in order to minimize forward movement of the passenger. While it is known to use pyrotechnic gas generators to operate mechanisms that wind up or otherwise pull in slack in the seat belt during a collision, such known pyrotechnic gas generators are often disposed internally of a seat belt retractor. Thus, the vehicle owner is faced with a significant cost penalty in that the entire pretensioner and retractor assembly must be replaced after activation because of the inability to prevent degradation of the retractor. High-temperature gases tend to abrade interior metal surfaces and produce ash and clinkers that bind up the retraction mechanism.
Further, where pyrotechnic gas generators are used to supply the rapidly increasing gas pressure, the gas pressurization rate and the resultant initial driving force or acceleration of the piston can exceed the structural capability of the driven components. As a result, the entire assembly can malfunction due to fracturing of a given part.
Another problem with known pretensioners is that they are designed to activate only in severe accidents, for example, accidents that exhibit inertial forces at least sufficient to activate the vehicle airbags. Safety system designers generally choose such a relatively high activation threshold due to the expense of replacing the entire seat belt retractor and pretensioner assembly after activation. As a result, seat belt pretensioners oftentimes may not protect passengers in less severe accidents. A related problem with known pretensioners is that when the pretensioner is activated only in severe accidents, activation will likely come relatively late in the crash sequence. Thus, the pretensioner must rapidly take up slack in the seat belt, sometimes injuring the passenger.
U.S. Pat. No. 5,967,440, herein incorporated by reference, describes a pretensioner and a conventional retractor for a safety belt system. The pretensioner is relatively complex and large, thus increasing the weight and spatial requirements of the seatbelt assembly.
U.S. Pat. No. 5,899,399, herein incorporated by reference, describes a state of the art pretensioner. The pretensioner is relatively complex, therefore complicating the manufacturing process.
U.S. Pat. No. 5,944,350, herein incorporated by reference, also describes a state of the art pretensioner. Again, the pretensioner is relatively large, thus increasing the weight and spatial requirements of the seatbelt assembly.
The present invention is directed to overcoming one or more of the problems or limitations associated with the prior art.
In one aspect of the invention, a seatbelt pretensioner incorporates a soft-start piston actuator. The seatbelt pretensioner cooperates with a webbing reel shaft axially disposed within a seatbelt retractor reel. The seatbelt pretensioner preferably includes a housing having a first passage in volumetric intersection with a second passage. The webbing reel shaft extends through the first passage. The second passage has a first end and a second end, the second end extending across and past the diameter of the first passage. A clutch assembly is also provided, and as seen in a preferred embodiment, is fixed within the first passage. The webbing reel shaft extends through the clutch assembly. The seatbelt pretensioner is further equipped with a strap that has a first end fixed to the housing, a portion extending across a section of the second passage, and a second end coiled about and fixed to the clutch assembly. Still further, the seatbelt pretensioner provides a soft-start piston actuator with at least two pressure surfaces, wherein the actuator is positioned adjacent the first end of the second passage. A gas generant composition is also provided that is ignitable to supply pressurized gas that initially acts upon one of the at least two pressure surfaces with a resultant force great enough to drive the actuator a predetermined distance. As the actuator is propelled forward, the pressurized gas subsequently exerts a greater resultant force on at least one additional pressure surface. As the actuator is driven forward, it tensions the strap, thus resulting in a peripheral force about the clutch assembly. A simultaneous rotary advancement of the clutch assembly, the webbing reel shaft, and the webbing reel therefore tensions the attached seatbelt webbing.
In a second aspect, a method of pretensioning a seatbelt is provided. The pretensioning method includes the steps of providing a housing with at least one passage, the passage having a first end and a second end, and providing a gas generator in fluid communication with the first end of the passage for providing a gas pressure thereto. The method also includes the steps of providing an actuator with at least a first and a second pressure surface, and positioning the actuator proximate the first end of the passage. The actuator and the passage are in flush communication at the first end. Further, the method includes the steps of operably coupling the actuator to a seatbelt retractor webbing reel with a strap or cord, and activating the gas generator to initially provide a gas pressure to the first pressure surface. Subsequently, the gas pressure is supplied to the second pressure surface of the actuator, thereby driving the actuator from the first end toward the second end of the passage. Driving the actuator in this fashion tensions the strap and winds the seatbelt retractor webbing reel with a more gradual increase in velocity, thereby pretensioning the seatbelt webbing thereon. By first driving the actuator at a lesser velocity, then incrementally increasing the velocity, by increasing the surface area of the actuator acted upon by gas pressure, damage to the various system components is avoided.